An Internal Ribosomal Entry Site (IRES) is a genetic element found in the genome of certain eukaryotic plus strand RNA viruses and cellular mRNAs. IRESs direct attachment of the 30S ribosomal subunit to a site downstream of the commonly used 5'terminal cap. IRES elements are large segments of RNA of different sequence that are highly structured. Neither the precise structure of these elements nor the precise mode of function is known. In this application, we propose to study the structure of the IRES of hepatitis C virus with respect structure and function. Furthermore, we will attempt to decipher the role of RNA binding proteins in the function of picornavirus and HCV IRES function. We have previously observed that an exchange of the poliovirus IRES element with that of rhinovirus type 2 results in a chimera, expressing an attenuated neurovirulence phenotype. We will investigate the molecular reason for this attenuation. Finally, we will determine whether we can exchange viral IRESs also IRESs of cellular mRNA.